1. Field of the Invention
The present invention relates to electrolytic cells for obtaining aluminum from aluminum chloride by electrolyzing a molten salt electrolytic bath containing molten aluminum chloride.
2. Description of the Prior Art
An aluminum chloride electrolyzing method wherein aluminum is obtained by electrolyzing a molten salt electrolytic bath containing molten aluminum chloride for example, an electrolytic bath of AlCl.sub.3 --NaCl--LiCl system or AlCl.sub.3 --MgCl.sub.2 --NaCl system, at a temperature above the melting point of aluminum has various advantages. Thus it can be operated at an electrolyzing temperature near the temperature of 700.degree. C. which is about 300.degree. C. lower than in the Hall Heroult process and that, as the anode reaction product by the electrolysis is a chlorine gas, no reaction with graphite used as an electrode material will take place and therefore the electrode will not be worn. This way of electrolytically producing aluminum therefore holds the possibility of saving energy and resources but is not yet established as an industrial process.
However, the most promising embodiment of this technique up to now is an electrolytic cell using horizontal bi-polar electrodes manufactured recently by ALCOA, U.S.A. (U.S. Pat. No. 3,822,195).
An important feature of this electrolytic cell of ALCOA is the provision of many horizontal rectangular graphite electrode plates between both electrodes of an electrolytic cell filled with a halide molten salt containing aluminum chloride so as to produce a proper clearance from the inner wall of the cell. In operation the aluminum chloride in the bath between the respective laminated electrodes is electrolyzed by passing an electric current between both electrodes so as to produce a chlorine gas between the anodes of the respective electrodes and molten aluminum grains on the cathode surfaces, so that the chlorine gas produced at the anodes is made to rise through the air gap formed between the electrodes and the inner wall of the cell as a rising passage from one side of the rectangular electrodes. The rising gas flow produces a unidirectional circulating current in the electrolytic bath while on the other hand, the molten aluminum grains produced at the cathodes decend on the cathode surfaces countercurrently against the circulating current through the gas rising passage under their own weight and are accumulated in the bottom of the cell.
However, in the ALCOA electrolytic cell, as described, it has been observed that, as the chlorine gas and molten aluminum move countercurrently through the same gas rising passage and the chlorine gas produced at the anodes concentrates on one side of the rectangle, the gas content in the electrolytic bath between the electrodes on the discharging side will be large and the chances of the aluminum and chlorine gas contacting each other will be so high that the aluminum tends to become re-chlorinated and the overall efficiency of the cell reduced.